Existing solar panel installations based on photovoltaic cells are susceptible to degradation over time. Because of the degradation the efficiency, thus the energy output, of these solar panel installations will diminish over time. Degradation of solar panels may have different causes such as physical damage, hot spots, snail trails, mismatch, delamination, back currents, potential induced degradation (PID), and other defects that are caused or deteriorated by potentials which accelerate the degradation of solar panels. Such defects are widespread, occurring in at least 75% of solar panel installations.
Physical damage may be due, among other things, to production errors during manufacturing of the solar panels, damage sustained during transport or incorrect installation of the solar panels, and external circumstances such as for example the weather while the solar panels are in use. Such defects may be significantly reduced by handling the solar panels with appropriate care, but can never be entirely prevented. Once present, such defects can also lead to further defects during the use of the solar panels.
Hot spots occur in solar panels of which some photovoltaic cells are located in the shade or are damaged. These photovoltaic cells present a higher resistance than the surrounding, illuminated and undamaged photovoltaic cells, which produce a higher current. This higher current also has to pass through the unilluminated and/or damaged cells with a higher resistance, causing them to heat up. As a consequence, these cells may sustain further damage, and cause damage to surrounding parts of the solar panel as well.
Mismatch is a similar problem to hot spots, in which solar panels have been manufactured with photovoltaic cells having different properties. As a consequence, certain photovoltaic cells may be under heavier load during use than others, and consequently sustain damage.
Hot spots and mismatches may be partly dealt with by placing a “bypass” diode in parallel with a number of photovoltaic cells. If the resistance of a particular set of photovoltaic cells then grows too high, the current from the other photovoltaic cells will pass through this bypass diode, such that no load is placed on the defect cells. However, this protective measure needs to be implemented during manufacture of the solar panels, and is not capable of remedying any causes of the defect.
Snail trails are dark discolorations observed on the surface of solar panels. These discolorations generally are a finger's width and similar in shape to a snail's mucus trail. This defect is most probably caused by chemical reactions occurring between the plastic material surrounding the photovoltaic cells and the conductive grid on the photovoltaic cells. This defect can be remedied by using different materials in which this phenomenon does not occur for the manufacture of solar panels. For existing solar panel installations, however, this is not a solution.
Delamination is the peeling off of the different protective layers applied to solar panels. This phenomenon is due, among other things, to external conditions such as for example humidity, fluctuations in temperature and sunlight. Improvements in the protective layers used may partly prevent this problem, but not entirely. The risk of delamination always remains, and once the defect appears it will increase and possibly cause further defects. Moreover, the use of improved protective layers is no solution for the solar panels of solar panel installations already in use.
Normally, an electrical current flows from the solar panels to the electricity grid or to a battery. The current may however flow in the other direction as well, from the electricity grid or a battery to the solar panels. These back currents can place the solar panels under load and cause damage. The solar panels can be protected against this by installing diodes that prevent currents flowing back, but these are effective only up to a certain breakdown voltage.
The potential induced degradation (PID) of solar panels is due to the electrons from the semiconductors of the photovoltaic cells flowing out to the surrounding structures, such as for example an overlying glass plate, an underlying support plate, the surrounding protective layers, and the frame or supporting structure in which the solar panel is mounted. This flowing out occurs due to the high voltages that build up between the photovoltaic cells and these structures. The flowing out of electrons influences the PN junction between the semiconductors in the photovoltaic cell, thereby reducing their function.
WO 2012/168249 A2 shows a method for detecting PID in photovoltaic cells or panels during the manufacturing process. It involves placing a conductive plastic plate against the front side or back side of a photovoltaic cell or panel and applying a DC voltage higher than 50V (up to, for example, 6500V) between both. An electrical characteristic (current-voltage characteristic or I-V characteristic, parallel resistance) of the photovoltaic cell or panel is then measured at different points in time to assess its quality and suitability. This test should preferably be performed under controlled conditions of temperature (preferably 85° C.) and atmospheric humidity (preferably 85%).
WO 2012/168250 A1 shows a similar method, divided into several steps to speed up the process. Well-functioning photovoltaic cells are quickly recognized in a first test phase, which can be performed quickly; only photovoltaic cells that perform badly are subjected to further tests and, optionally, regeneration, to carry out a further selection. The first test should preferably be performed under controlled conditions of temperature (>60° C., preferably 85° C.) and atmospheric humidity (>60%, preferably 85%). Further tests may be performed under different conditions to simulate day and night. Voltages of 0V up to −1000V are applied.
A disadvantage of these methods is that they require a special sensor to be applied over the front side or the back side of the photovoltaic cell or panel. A sensor that is suited for all types and sizes of solar panels is difficult to produce. Another disadvantage is that these methods need to be carried out under controlled conditions and are therefore not suitable for testing installed solar panels in an operating environment.
CN 102864439 A shows a method for preparing an antireflection film that is resistant against the PID effect. This antireflection film can protect the photovoltaic cells in a solar panel against PID, but offers no improvement of the problem once it has occurred. An additional disadvantage of the antireflection film is that it cannot be applied to existing solar panels.
CN 102 565 658 discloses a test method of PID (Potential-Induced Degradation) of a solar cell module. The test method comprises the following steps of: (1) testing and recording initial data of a tested solar cell module; (2) installing the tested solar cell module in a high-temperature and low-temperature experimental environment box and carrying out insulated treatment between the tested solar cell module and the high-temperature and low-temperature experimental environment box; (3) polarly connecting the anode and the cathode of the tested solar cell module, which are subjected to short-circuited connection, with the cathode of high-voltage loading equipment, and connecting a frame of the solar cell module with the anode of the high-voltage loading equipment; (4) starting the high-temperature and low-temperature experimental environment box, starting the high-voltage loading equipment and debugging the high-voltage loading equipment to the output voltage value of 600-1000V, and simultaneously starting a current monitor for carrying out electric leakage monitoring; (6) testing and recording final data of the tested solar cell module; (7) comparing the initial data with the final data of the tested solar cell module and evaluating power degradation; and (8) finishing the test.
In “Crystalline Si solar cells and modules featuring excellent stability against potential-induced degradation” by H. Nagel et al., from the “26th EUROPEAN INTERNATIONAL CONFERENCE ON PHOTOVOLTAIC SOLAR ENERGY 5-9 Sep. 2011, HAMBURG, GERMANY”, 9 Sep. 2011, pages 3107-3112, an assessment of all kinds of solutions for potential-induced degradation (PID) of p-type crystalline silicon solar cells reveals that a great demand exists on i) PID-resistant solar cells and ii) on alternative encapsulation materials which protect PID-prone cells in the module. A further assessment of solutions for PID is disclosed describing that a positive voltage between modules and ground may be applied for regeneration at night.
In PINGEL S ET AL, “Potential Induced Degradation of solar cells and panels”, 35TH IEEE PHOTOVOLTAIC SPECIALISTS CONFERENCE (PVSC), 20-25 Jun. 2010, HONOLULU, Hi., USA, IEEE, PISCATAWAY, N.J., USA, 20 Jun. 2010, pages 2817-2822, it is disclosed that solar energy generation is getting more and more important worldwide PV systems and solar parks are becoming larger consisting of an increasing number of solar panels being serially interconnected. As a consequence panels are frequently exposed to high relative potentials towards ground causing High Voltage Stress (HVS). The effect of HVS on long term stability of solar panels depending on the leakage current between solar cells and ground has been first addressed by NREL in 2005 [1]. This potential degradation mechanism is not monitored by the typical PV tests listed in IEC 61215 [2]. Depending on the technology different types of Potential Induced Degradation (PID) occur. This paper is focusing on PID of wafer based standard p-type silicon technology aiming on increasing life times for solar panels once exposed to external potentials in the field. A test setup is presented for simulation of the PID in the lab and the influence of cell properties on PID is demonstrated in order to reveal the cell being the precondition for the PID. The paper further discloses that by grounding of the positive pole of the PV system and thereby avoiding of harmful potentials leads to regeneration of affected solar panels. This recovery process takes time and the rate depends on the potential and environmental factors such as humidity and temperature.
Solutions already exist, then, that can detect of prevent the different defects in solar panels. However, some of these solutions require adjustments to the solar panels themselves and are therefore impossible or very hard to implement in existing solar panel installations. These solutions also require the manufacturing process of solar panels to be modified and will increase the production cost of these solar panels. Other solutions are very laborious to implement in existing solar panel installations and cannot be expanded to all types of solar panel installations in a straightforward manner. Moreover, most of these solutions are intended to either detect or prevent the defects.
It is an aim of the present invention to prevent or at least mitigate one or more of the problems described above, and/or to provide improvements in general.